1. Field of the Invention
The present invention relates to a semiconductor device and a method of controlling the semiconductor device, and more particularly, to a semiconductor device that reads data by using sense amplifiers, and a method of controlling the semiconductor device.
2. Description of Related Art
A DRAM (Dynamic Random Access Memory) as an example of a semiconductor device stores data through charges stored in memory cells. The memory cells are placed at the intersection points between word lines and bit lines. The data reading operation in such a DRAM is performed as follows. First, a word line is activated. The memory cells connected to the activated word line are electrically connected to the bit lines. The potentials of the bit lines are slightly changed by the electric charges (data) stored in the memory cells. The sense amplifiers connected to the bit lines are then activated, to amplify the potential difference between each pair of bit lines. After that, the bit lines are electrically connected to a data input/output circuit via column switches. Through the potentials amplified by the sense amplifiers, the data input/output circuit recognizes the existence of electric charges stored in the memory cells or the 1-bit data stored in each memory cell. After that, the word line is inactivated. Hereinafter, the period from inactivation of an activated word line to activation of another word line will be referred to as a “unit time” or a “cycle time.”
Most of the internal circuits in a DRAM are normally driven by an internal voltage that is stabilized by lowering an external voltage supplied from outside. The internal voltage is substantially fixed, regardless of fluctuations of the external voltage. According to Japanese Patent Application Laid-Open Nos. 2003-196977 and 2010-146675, the active timeout period indicating the period from activation of a word line and activation of sense amplifiers to resetting of the word line varies between an external access mode and a refresh mode. Japanese Patent Application Laid-Open No. 11-297650 discloses a method of controlling overdriving in a circuit that generates signals for activation and inactivation of word lines and sense amplifiers. The signal generating circuit includes circuits that operate at an internal voltage and circuits that operate at an external voltage.
To shorten the unit time, the sensitivity of the sense amplifiers needs to be increased, and the period of time required for amplifying a very small voltage to a predetermined value needs to be shortened.
Therefore, when sense amplifiers are activated, an external voltage higher than an internal voltage is temporarily supplied to the high-potential side of each sense amplifier. In this manner, the sense amplifiers are overdriven, and the sensitivity of the sense amplifiers is increased. After that, the control circuit for the sense amplifiers inactivates the overdriving, and the sense nodes of the sense amplifiers almost reach the internal voltage at the end. The overdrive period (or the end of the overdrive time) greatly affects the period of time required by the bit lines to reach a predetermined potential. The active timeout period for inactivating a word line under the condition that the bit lines reach a potential that is 98% of the internal voltage also relates with the end of the overdrive time. The timing to inactivate a word line is normally determined by the control circuit that defines the overdrive time. The external voltage is higher than the internal voltage, but is not as stable as the internal voltage. Therefore, when the external voltage is high, the overdrive period is set relatively short, and when the external voltage is low, the overdrive period is set relatively long. In this manner, stable overdriving can be performed. The external voltage is supplied to the control circuit that defines the overdrive period, and the control circuit outputs a signal indicating the period of time that varies with fluctuations of the external voltage.
The trigger of inactivation of a word line is based on the overdrive period. However, if the overdrive period varies with the external voltage, the effective length of the unit time or cycle time (the actual value of the unit time) greatly depends on the external voltage. As a result, the unit time to be secured is not easily estimated. Specifically, the unit time is determined based on the semiconductor device and the controller that controls the semiconductor device. When the external voltage is particularly low, the actual value of the unit time may be too short with respect to a specified value. This matter becomes particularly conspicuous when “units time” are repeatedly set in response to refresh commands from outside.